Appendix C – Electrical Interface
Sealevel Systems COMM+4.PCI Page 13
Appendix C - Electrical Interface
RS-232
Quite possibly the most widely used communication standard is RS-232. This
implementation has been defined and revised several times and is often referred
to as RS-232-C/D/E or EIA/TIA-232-C/D/E. It is defined as “Interface between
Data Terminal Equipment and Data Circuit- Terminating Equipment
Employing Serial Binary Data Interchange”. The mechanical implementation of
RS-232 is on a 25-pin D sub connector. The IBM PC computer defined the
RS-232 port on a 9 pin D sub connector and subsequently the EIA/TIA
approved this implementation as the EIA/TIA-574 standard. This standard has
defined as the “9-Position Non-Synchronous Interface between Data Terminal
Equipment and Data Circuit-Terminating Equipment Employing Serial Binary
Data Interchange”. Both implementations are in wide spread use and will be
referred to as RS-232 in this document. RS-232 is capable of operating at data
rates up to 20K bps / 50 ft. The absolute maximum data rate may vary due to
line conditions and cable lengths. RS-232 often operates at 38.4K bps over very
short distances. The voltage levels defined by RS-232 range from -12 to +12
volts. RS-232 is a single ended or unbalanced interface, meaning that a single
electrical signal is compared to a common signal (ground) to determine binary
logic states. A voltage of +12 volts (usually +3 to +10 volts) represents a binary
0 (space) and -12 volts (-3 to -10 volts) denote a binary 1 (mark). The RS-232
and the EIA/TIA-574 specification define two types of interface circuits Data
Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE).
The Sealevel Systems Adapter is a DTE interface.
RS-422
The RS-422 specification defines the electrical characteristics of balanced
voltage digital interface circuits. RS-422 is a differential interface that defines
voltage levels and driver/receiver electrical specifications. On a differential
interface, logic levels are defined by the difference in voltage between a pair of
outputs or inputs. In contrast, a single ended interface, for example RS-232,
defines the logic levels as the difference in voltage between a single signal and a
common ground connection. Differential interfaces are typically more immune
to noise or voltage spikes that may occur on the communication lines.
Differential interfaces also have greater drive capabilities that allow for longer
cable lengths. RS-422 is rated up to 10 Megabits per second and can have
cabling 4000 feet long. RS-422 also defines driver and receiver electrical
characteristics that will allow 1 driver and up to 32 receivers on the line at once.
RS-422 signal levels range from 0 to +5 volts. RS-422 does not define a
physical connector.